add header comment with example
bug fix: bird track comments can start with spaces prior to the bird tracks
bug fix: include Diagrams.Backend.SVG in modules for diagrams-builder
add .diagrams-cache directory to .gitignore
minor whitespace cleanup
fiddle with output file location etc.
It works!!!

Some of the changes in this commit were created by diagrams-haddock
itself!  Also, switch to a custom build system which copies over
images as a post-haddock hook, as explained here:

  https://github.com/diagrams/diagrams-doc/issues/2#issuecomment-13794463